Method and Circuit Intended for High-Frequency Communication Between an Interrogator and a Smart Tag

1. A method intended for high-frequency communication between an interrogator and a smart tag, a circuit of said smart tag being galvanically coupled with a voltage source and, according to which method the smart tag observes a first phase (Φi), which is a phase of a voltage induced in a tag's antenna by a high-frequency carrier signal generated by the interrogator, characterized in that the smart tag observes the first phase (Φi) in those time intervals located even within a data frame being transmitted, in which time intervals, according to a communication protocol, the smart tag does not transmit packets of high-frequency radio waves, and that the smart tag transmits said high-frequency wave packets, in that it excites its own antenna with a voltage, the phase of which voltage, being a second phase (Φt), is set, at the beginning of transmitting each said high-frequency wave packet, each time shifted with respect to said observed first phase (Φi) by the same phase angle (ΔΦ) being determined by a relation ΔΦ=Φt−Φi.

2. The method as recited in claim 1 , characterized in that the smart tag observes the first phase (Φi) in the time intervals with a duration according to the communication protocol being long enough that the smart tag can match the second phase (Φt) with a phase, which is shifted by the phase angle (ΔΦ) with respect to the first phase (Φi).

3. The method as recited in claim 2 , characterized in that said time intervals of the phase observation are placed closely before the instants, in which, according to the communication protocol, the smart tag starts transmitting said high-frequency wave packets.

4. The method as recited in claim 3 , characterized in that said time intervals of the phase observation are placed in a time window with no tag's transmission even within the duration of bits, in which bits, according to the communication protocol, the smart tag transmits said high-frequency wave packets.

5. The method as recited in claim 4 , characterized in that the smart tag transmits said high-frequency wave packets, in that it excites its own antenna with a simple harmonic voltage, the phase of which voltage, being a second phase (Φt), is set, at the beginning of transmitting each said high-frequency wave packet, shifted each time by the same said phase angle (ΔΦ).

6. The method as recited in claim 4 , characterized in that the smart tag transmits said high-frequency wave packets, in that it excites its own antenna with a voltage having a shape of a pulse group and, the phase of which voltage, being a second phase (Φt), is set, at the beginning of transmitting each said high-frequency wave packet, shifted each time by the same phase angle (ΔΦ).

7. The method as recited in claim 5 , characterized in that the frequency (ft) of the radio waves in said tag's high-frequency wave packets is equal to the frequency (fi) of the interrogator's carrier signal and that said phase angle (ΔΦ) is such that an amplitude of voltage across an interrogator's antenna each time when some of said wave packets influence this antenna always attains the largest attainable interference rise or the largest attainable interference drop.

8. The method as recited in claim 7 , characterized in that the largest attainable interference rise in the amplitude of the voltage across the interrogator's antenna is attained by setting said second phase (Φt) shifted with respect to said observed first phase (Φi) by the phase angle being ΔΦ=180°.

9. The method as recited in claim 7 , characterized in that the largest attainable interference drop of the amplitude of the voltage across the interrogator's antenna is attained by setting said second phase (Φt) shifted with respect to said observed first phase (Φi) by the phase angle being ΔΦ=0.

10. The method as recited in claim 1 , characterized in that the smart tag should set said second phase (Φt) with respect to said observed first phase (Φi) with an accuracy better than 20° and preferably 10°.

11. The method as recited in claim 8 , characterized in that the smart tag selects a value of said phase angle (ΔΦ) and said second phase (Φt) is then set automatically.

12. The method as recited in claim 11 , characterized in that in said time intervals of the phase observation the smart tag digitizes a signal (rs′), which the interrogator's carrier signal induces in the tag's antenna and is then amplified, and that said digitized received signal (drs) is used as a reference signal for a phase-frequency comparator.

13. The method as recited in claim 12 , characterized in that the phase-frequency comparator is connected into a phase-locked loop, whose voltage-controlled oscillator operates at the frequency (fi) of the interrogator's high-frequency carrier signal and whose output signal is shifted by the phase angle ΔΦ+90° and is amplified and conducted to the tag's antenna through a capacitor as a simple harmonic signal to be transmitted in the form of high-frequency wave packets.

14. The method as recited in claim 12 , characterized in that the phase-frequency comparator is connected into a phase-locked loop whose voltage-controlled oscillator operates at a frequency, which is a multiple of the frequency (fi) of the interrogator's high-frequency carrier signal, and an output signal of said voltage-controlled oscillator is conducted through a digital-to-analogue converter or a digital divider, where the phase is shifted by the phase angle ΔΦ+90° and is amplified and conducted to the tag's antenna through a capacitor as a simple harmonic signal to be transmitted in the form of the high-frequency wave packets.

15. The method as recited in claim 14 , characterized in that the voltage-controlled oscillator operates at a frequency, which is an eighthfold, preferably a sixteenthfold of the frequency (fi) of the interrogator's high-frequency carrier signal.

16. The method as recited in claim 13 , characterized in that the phase-locked loop opens while the interrogator interrupts transmitting the high-frequency carrier signal.

17. The method as recited in claim 13 , characterized in that in said time interval of the phase observation the smart tag observes a first amplitude (Ai), which is an amplitude of the voltage induced in the tag's antenna by the interrogator's high-frequency carrier signal, and that the phase-locked loop opens whenever said first amplitude (Ai) drops below a certain value.

18. The method as recited in claim 13 , characterized in that the phase-locked loop opens closely before the instants, in which, according to the communication protocol, the smart tag starts transmitting said high-frequency wave packets.

19. The method as recited in claim 16 , characterized in that the smart tag uses the output signal of said voltage-controlled oscillator as a clock signal, which oscillator at that time operates either in the closed phase-locked loop mode or in the open phase-locked loop mode.

20. The method as recited in claim 1 , characterized in that the smart tag observes the first amplitude (Ai), which is an amplitude of the voltage induced in the tag's antenna by the interrogator's high-frequency carrier signal, and that the smart tag transmits high-frequency wave packets in that it excites its own antenna with the voltage, an amplitude of which voltage is essentially constant during transmitting each said high-frequency wave packet and as a second amplitude (At) it is mainly set with respect to the first amplitude (Ai) observed by then so that the second amplitude (At) is inversely proportional to the first amplitude (Ai).

21. The method as recited in claim 20 , characterized in that the smart tag observes the first voltage amplitude (Ai) in time intervals, in which, according to the communication protocol, the smart tag does not transmit said high-frequency wave packets.

22. The method as recited in claim 21 , characterized in that the tag's antenna is excited by the voltage, whose second amplitude (At) is automatically set to a highest value (Atmax) when the first amplitude (Ai) is below its reference value (Airef), and that the tag's antenna is excited by the voltage, whose second amplitude (At) is automatically set to a value, which is determined by an expression Atmax·Airef/Ai when the first amplitude (Ai) is above its reference value (Airef).

23. The method as recited in claim 22 , characterized in that the reference value (Airef) of the first amplitude (Ai) is determined as a twofold to fivefold of such minimum value (Aimin) of the first amplitude (Ai), which the interrogator's magnetic field induces in the tag's antenna at a lowest value, as required by the standard, of the magnetic field density at a position of the tag's antenna.

24. The method as recited in claim 1 , characterized in that the resonance frequency (fi) of the interrogator's antenna circuit equals 13.56 MHz.

25. The method as recited in claim 24 , characterized in that the length of duration of said high-frequency wave packets equals the length of duration, as required by the standard, of a load-modulation of antenna's impedance in passive smart tags.

26. A circuit intended for high-frequency communication between an interrogator and a smart tag a circuit of said smart tag being galvanically coupled with a voltage source and a received signal (rs) induced in a tag's antenna (A) being conducted to an input of a variable-gain amplifier (VGA) and an output of said variable-gain amplifier (VGA) being connected through a digitizer (Dig) or directly to a reference input of a phase-matched signal generator (PhMSG; PhMSG′) and the smart tag transmitting high-frequency radio wave packets, which are generated out of a phase-matched signal (ts′) from an output of the phase-matched signal generator (PhMSG; PhMSG′), characterized in that each time at the beginning of transmitting each said wave packet said phase matched signal (ts′), whose phase is a second phase (Φt) increased by 90°, gets phase-matched in the phase-matched signal generator (PhMSG; PhMSG′) with said received signal (rs), which an interrogator's high-frequency carrier signal induced in the tag's antenna (A) and whose phase as a first phase (Φi) is observed in said phase-matched signal generator (PhMSG; PhMSG′), so that the second phase (Φt) is shifted with respect to said observed first phase (Φi) by the same phase angle (ΔΦ) being determined by a relation ΔΦ=Φt−Φi, wherein the second phase (Φt) is a phase of a voltage exciting the tag's antenna (A) to transmit said high-frequency wave packets, that the output of the phase-matched signal generator (PhMSG; PhMSG′) is connected to the tag's antenna (A) through an output amplifier (OA) and a capacitor, which output amplifier sets an amplitude of the voltage across the tag's antenna (A) as a second amplitude (At) to form said high-frequency wave packets, and that, in order to form said high-frequency wave packets, the phase-matched signal generator (PhMSG; PhMSG′) and the output amplifier (OA) are controlled by a transmit-on signal (tos) defining the start and end of tag's transmitting.

27. The circuit as recited in claim 26 , characterized in that a frequency (ft) of the radio waves in said tag's high-frequency wave packets is equal to a frequency (fi) of the interrogator's high-frequency carrier signal and that said phase angle (ΔΦ) is such that an amplitude of the voltage at an interrogator's antenna at the time when some of said wave packets influence this antenna always attains the largest attainable interference rise or the largest attainable interference drop.

28. The circuit as recited in claim 26 , characterized in that the phase-matched signal generator (PhMSG) comprises a phase-frequency comparator (PhFC), to whose reference input a digitized received signal (drs) is conducted and whose output is conducted to a phase-locked voltage-controlled oscillator (VCO), which operates at the frequency (fi) of the interrogator's high-frequency carrier signal and whose output is connected to a phase shifter (PhSh), which shifts the phase of an input signal by said phase angle (ΔΦ) increased by 90° and whose output signal is said phase matched signal (ts′), wherein the phase-frequency comparator (PhFC), the voltage-controlled oscillator (VCO) and the phase shifter (PhSh) are controlled by said transmit-on signal (tos).

29. The circuit as recited in claim 26 , characterized in that the phase-matched signal generator (PhMSG′) comprises a phase-frequency comparator (PhFC), to whose reference input a digitized received signal (drs) is conducted and whose output is conducted to a phase-locked voltage-controlled oscillator (HFVCO) with a frequency divider (FDiv), which voltage-controlled oscillator (HFVCO) operates at an eighthfold, preferably a sixteenthfold of the frequency (fi) of the interrogator's high-frequency carrier signal and whose output is connected either to an input of a digital-to-analogue converter (DAC) or to an input of a digital divider (DigDiv), wherein by a digital presetting of the digital-to-analogue converter (DAC) or the digital divider (DigDiv) the phase of an input signal is shifted by said phase angle (ΔΦ) increased by 90°, and whereby said phase matched signal (ts′) is generated, wherein the phase-frequency comparator (PhFC), the voltage-controlled oscillator (HFVCO) and the digital-to-analogue converter (DAC) or the digital divider (DigDiv) are controlled by said transmit-on signal (tos).

30. The circuit as recited in claim 28 , characterized in that the generator (PhMSG; PhMSG′) of the phase-matched signal (ts′) sets said second phase (Φt) with respect to said observed first phase (Φi) with an accuracy better than 20° and preferably 10°.

31. The circuit as recited in claim 30 , characterized in that a phase-shift control signal (pscs) is conducted to a control input of the phase shifter (PhS) or a control input of the digital-to-analogue converter (DAC) or the digital divider (DigDiv).

32. The circuit as recited in claim 31 , characterized in that an amplitude measuring circuit (AMC) observes an amplitude of the received signal (rs) as a first amplitude (Ai), which is an amplitude of the voltage induced in the tag's antenna (A) by the interrogator's high-frequency carrier signal, and with its first output signal (avs) representing a measured amplitude value controls said output amplifier (OA) in a way that said second amplitude (At), which is the amplitude of the voltage across the tag's antenna (A) to form a new wave packet, is set with respect to first amplitude (Ai) as observed each time by then and with its second output signal (ads) representing the measured amplitude-decrease value controls said phase-frequency comparator (PhFC) in a way that said phase-locked loop opens whenever said first amplitude (Ai) drops below the predetermined value.

33. The circuit as recited in claim 32 , characterized in that the smart tag sets a gain of the variable-gain amplifier (VGA) by means of a gain control signal (gcs).

34. The circuit as recited in claim 33 , characterized in that the variable-gain amplifier (VGA) and the amplitude measuring circuit (AMC) are connected to the tag's antenna (A) through an attenuator and a DC voltage defining circuit (Att/DC) and that said attenuator and DC voltage defining circuit (Att/DC) is controlled by means of said transmit-on signal (tos) defining the start and end of the tag's transmission so that said circuit attenuates the signal from the tag's antenna (A) while the smart tag transmits the high-frequency wave packets and sets up a DC voltage level while the smart tag does not transmit the high-frequency wave packets.